Ultra high q superconductive cavity resonator made of niobium having a limited number of crystal grains



Feb. 3, 1 970 I. WEIS$MAN ULTRA HIGH Q SUPERGONDUCTIVE CAVITY RESONATOR MADE OF NIOBIUM HAVING A LIMITED NUMBER OF CRYSTAL GRAINS Filed Dec. 21, 1967 FIG.I l3 5 "f2 1 I BEAM- SOURCE OF 4 BEAM g Q Q 3v PARTICLES i y 6 V |2\ cmrosm' 7 men VACUUM PUMP FIG.2 l6 I9 5, ART FIG.6 i Q I53 gm A I084" TEo f 2 X-BAND a NIOBIUM I: lulu-1| "I" 'y z I II I I0 I09 I 8 0 QdQJNLOADEDHQDG SCALE) FORM THE ANNEALTO GROW CAVITY LARGE GRAINS & RESONATORS RELIEVE STRESS (c) (a) ELECTRO-POLISH ASSEMBLE & TUNE ACCELERATOR FIG.4

INVENTOR.

WEISSMAN United States Patent O US. Cl. 315-35 3 Claims ABSTRACT OF THE DISCLOSURE A superconductive linear microwave particle accelerator is disclosed. The accelerator employs a coupled cavity accelerating structure arranged along a beam path for electromagnetic interaction with a beam of charged particles for accelerating the particles to nearly the velocity of light at the downstream end of the accelerator. The coupled cavity accelerator section comprises a plurality of coupled superconductive cavities made of niobium. The walls of the cavity resonators, which define the microwave current supporting surfaces of the cavities, are constituted of pure niobium having less than 20 crystallites of niobium per square inch as averaged over the entire current supportive surface of each cavity, whereby the radio frequency energy loss associated with the grain boundaries in the surface of the cavity is minimized in use. When there are less than 20 crystallites of niobium per square inch of the cavity surface, a Te mode X-band niobium cavity will exhibit Qs in excess of 5X10 at temperatures of 1.2 to 1.3 degrees Kelvin and such high Qs have been measured for this mode and frequency. When an accelerator section is formed ofcavities having less than 20 crystallites of niobium per square inch the Q of the coupled cavity accelerator section becomes extremely high at the operating temperatures of 1.2 to 1.3 degrees Kelvin, thereby greatly reducing the circuit energy losses caused by currents flowing in the accelerator walls and permitting relatively high current beams to be accelerated to nearly the velocity of light. Ultra high Q cavities, when dimensioned and excited in the high Q circular electric modes, may be employed as frequency determining elements of frequency standards. In one method according to the present invention, the ultra high Q cavities are first fabricated of niobium material having relatively small grain size. Such material may be readily cold worked without fracture. The fabricated accelerator or cavity structure is then annealed by heat treatment to cause the crystallites to grow in size and to relieve stresses in the material. The heating is continued until the crystallites grow to such a size that there are less than 20 of such crystallites per square inch as averaged over the entire current supportive surface of the cavity. The processed cavity is then electro-polished and tuned.

DESCRIPTION OF THE PRIOR ART Heretofore, niobium superconductive coupled cavity accelerator sections and single cavities have been proposed. However, in such structures the grain size of the niobium crystallites, constituting the superconductive surface of the cavity or cavities, has been relatively small such that there were greatly in excess of 20 crystallites per square inch as averaged over the current supportive surface of the cavity. The unloaded Qs for such prior art cavity structures have not been sufiiciently high to provide a practical linear accelerator or frequency standard component. Therefore, a need exists for improving the Q of niobium superconductive cavities and superconductive circuits employing a plurality of such coupled cavities.

3,493,809 Patented Feb. 3, 1970 "ice SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved niobium superconductive cavity structure and superconductive microwave circuits employing such cavities.

One feature of the present invention is the provision, in an ultra high Q superconductive microwave circuit, of a superconductive cavity resonator having the current supportive surface of the resonator formed of niobium having less than 20 crystallites of niobium per square inch as averaged over the entire current supportive surface of the cavity, whereby the radio frequency energy losses associated with grain boundaries in the surface of said cavity are minimized in use.

Another feature of the present invention is the'sarne as the preceding feature wherein the ultra high Q superconductive circuit comprises a coupled cavity microwave linear particle accelerator section including a plurality of superconductive cavities coupled together and arranged for interaction with a beam of charged particles, such plurality of cavities having their microwave current sup portive wall surfaces constituted of niobium having less than 20 crystallites per square inch as averaged over the current supportive surfaces of the cavities, whereby an ultra high Q superconductive accelerator section is obtained.

Another feature of the present invention is the method for fabricating an ultra high Q superconductive cavity including the steps of, forming the radio frequency current conductive wall surfaces of the cavity of niobium material, and heat treating the cavity to anneal the niobium material and to produce growth of the niobium crystallites such that the crystallites are reduced in number to less than 20 per square inch as averaged over the current supporting wall surfaces of the cavity.

Other features and advantages of the present invention will become apparent upon perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic longitudinal sectional view, partly in block diagram form, of a microwave superconductive linear particle accelerator incorporating features of the present invention,

FIG. 2 is an enlarged cross sectional view of a portion of the structure of FIG. 1 delineated by line 22,

FIG. 3 is a sectional view of the structure of FIG. 2 taken along line 33 in the direction of the arrows,

FIG. 4 is a flow diagram, in block diagram form, depicting a method of fabricating ultra high Q superconductive cavity circuits,

FIG. 5 is a perspective view of an ingot of niobium material having relatively large crystallites, and

FIG. 6 is a plot of the number ef niobium grains per square inch versus the unloaded Q for an X-Band TE mode niobium cavity operated at a temperature of 1.2 to 1.3 degrees Kelvin.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown a microwave linear particle accelerator 1 incorporating features of the present invention. The accelerator 1 includes a superconductive microwave accelerator section 2 formed by an array of coupled cavity resonators 3 successively arranged along a beam path 4 for electromagnetic interaction between the fields of the coupled cavity accelerator section 2 and the charged particles of the beam to accelerate the charged particles to nearly the velocity of light at the downstream end of the accelerator 2. A source 5 of beam particles such as ions or electrons is disposed at the upstream end of the accelerator for forming and projecting the beam of charged particles into the accelerator section 2. A particle permeable gas tight beam exit window -6 seals off the downstream end of the accelerator section 2. A suitable exit window material is a thin aluminum foil. A high vacuum pump 7 is connected into the accelerator envelope via an exhaust tubulation 8 for evacuating the beam source 5 and accelerator 2 to a suitably low pressure as of 10- torr.

A microwave source 9, for example, a high power klystron amplifier supplies microwave energy to the accelerator section 2 via a waveguide 11 and gas tight wave permeable microwave window 12 sealed across the waveguide 11. A cryostat 13 surrounds the accelerator section 2 for containing a cryogenic fluid such as liquid helium for cooling the accelerator section 2 to a superconductive temperature. A solenoid 14 surrounds the accelerator 2 and cryostat 13 for producing an axially directed beam focusing magnetic field along the beam path 4 for confining the beam to a desired diameter throughout the length of the accelerator 2.

Referring now to FIGS. 2 and 3, the accelerator section 2 is shown in greater detail. A suitable accelerator structure 2 is a disc loaded Waveguide structure of the biperiodic type operated in a resonant 1r/ 2 mode. Such a superconductive accelerator structure is described by Weaver et al. in an article titled, Accelerating Structures for Superconducting Electron Linacs, appearing in the IEEE Transactions on Nuclear Science, June 1967, pp. 345-349.

Briefiy, the bi-periodic disc loaded resonant accelerator section 2 includes a cylindrical waveguide 16 having a plurality of washer-shaped discs 17 axially spaced apart along the length of the cylindrical waveguide 16 to produce a disc loaded waveguide structure. The discs 17 are axially spaced apart in a bi-periodic manner to define a series of cylindrical cavity resonators 3 with every other one of the cavity resonators 3" having an axial length approximately one-half the axial length of the other resonators 3. The central holes 18 in the washer shaped discs 17 define a cylindrical beam passageway throughout the length of the accelerator section 2 for passage of the beam therethrough. In addition, the beam hole 18 provides means for capacitively coupling the cavities 3 and 3' together to form a coupled cavity accelerator structure 2. Accelerator structure 2 is preferably operated in a resonant mode corresponding to the 1r/2 mode wherein there is a 90 phase shift in the resonant electric field between adjacent resonators 3 and 3, respectively, throughout the length of the accelerator section 2.

The accelerator section 2 is immersed in a suitable cryogenic fluid such as liquid helium for cooling the accelerating structure 2 to between 1.2 and 1.3 degrees Kelvin. The cryostate 13, in which the accelerator section 2 is immersed, contains an inner chamber 19 containing the liquid helium cryogenic fluid. An evacuated hollow cylindrical chamber 21 surrounds the inner chamber 19 to minimize conduction of heat from the surrounding environment to the inner chamber 19. Another chamber 22 surrounds the evacuated chamber 21 and contains a second cryogenic fluid as of liquid nitrogen.

The disc loaded superconductive accelerator section 2 is made of niobium with the inside microwave current supporting surfaces of the cavities 3 and 3 being constituted of niobium material having the least number of crystallites of niobium per square inch as averaged over the entire current supportive surface of the cavities such that the radio frequency losses associated with grain boundaries in the surfaces of the cavities 3 and 3' are minimized in use. It has been found that grain boundaries in the surfaces of the cavity resonators 3 and 3' present a gross imperfeqtiqn in the supercondutcive surface and as such they can act as scattering sites for the conversion of pairs of superconducting electrons, i.e., Cooper pairs into normal electrons. In addition, impurities will concentrate at grain boundaries and provide sites for the generation of thermal EMFs which act to produce trapped flux during the normal to superconducting transition. Such trapped flux has a normal region associated with it which is lossy.

To achieve the ultra high Q suitable for a superconductive microwave linear accelerator section 2, or in the case of single superconductive cavities utilized for frequency standard applications, it is necessary to reduce the grain boundary density to some minimum allowable fraction of the total surface. It has been found that, if the current supportive surfaces of each cavity are constituted by less than 20 crystallites of niobium per square inch as averaged over the entire current supportive surface of the cavity, the ultra high Qs necessary for a practical accelerator section or as a practical reference cavity in a frequency standard are obtained at the lowest practicable operating temperature for the microwave structure such as 1.2 to 1.3 degrees Kelvin.

For example, it has been found that the Q of a single cylindrical cavity resonator, operating in the TE mode at X-band and having a length of 1.084 inches and an inside diameter of 1.53 inches, has an unloaded Q which varies with the number of niobium grains per square inch as plotted by line 25 in the plot of FIG. 6. The values of Q are for the cavity operating in the temperature range of 1.2 to 1.3 degrees Kelvin. Heretofore, the highest Q obtained by a prior art cavity structure for the aforecited conditions of temperature, frequency and mode was 3 10 and was obtained by a cavity mode of lead plated on copper having thousands of grains per square inch as averaged over the entire inner surface of the cavity resonator. This prior art cavity Q is indicated by the position of the vertical line 26 on the graph of FIG. 6. The unloaded Q of the prior art cavity falls just short of being practical for a microwave linear accelerator or :for areference cavity of a frequency standard.

It has been found that the Q of a niobium cavity can be substantially increased by reducing the number of crystallites forming the current supportive surface of the cavity resonator structure. More particularly, it has been found that if the crystallites are reduced in number to less than 20 per square inch, when averaged over the entire internal current supportive surface of the cavity, that the unloaded Q of the cavity can be increased to at least 5x10 thereby becoming practical as a reference cavity for a frequency standard or as a coupled cavity accelerator section of a superconductive accelerator. In one example of the aforecited niobium X-band cavity, the Q is increased to 1.3 X 10 by reducing the number of crystallites constituting the current supportive surface of the cavity to approximately 1.2 crystallites per square inch. It is contemplated that further increases in unloaded Q of the cavity can be obtained by reducing still further the number of crystallites constituting the current supportive surface of the cavity resonator structure.

Referring now to FIG. 4 there is shown, in a block diagram form, the steps of a method for fabricating cavities and coupled cavity accelerator sections of ultra high Q and having the superconductive current supportive surfaces of the cavity constituted by less than 20 crystallites of niobium per square inch as averaged over the entire current supportive surface of the cavity. According to the method of FIG. 4, in step (a), one or more cavity resonators are formed as by machining, hydroforming, or spinning pure niobium material having relatively small grains. These small grains are preferably equiaxed grains and on the order of 0.005 centimeters or less across for fabrication techniques requiring a substantial amount of cold working of the bulk material such as encountered in hydroforming and spinning. If the cavity is formed by machining, the grain, size can. be, arbitrarily large but the cold worked surface must subsequently be removed by processes such as electropolishing or annealing.

A relatively small grain niobium material is commercially available from WahChang Corporation of Albany, Oregon. Such small grain material is obtained by electron beam melting of niobio-m metal in a vacuum or by are melting or by a combination of the two melting processes, \following any or all of these by subsequent cold working and recrystallization.

In step (b), the fabricated cavity resonator structures are annealed by heat treatment in high vacuum, i.e., torr or lower for two hours at 2000 C. The annealing step serves to relieve mechanical stresses in the material and to also cause the grains to grow in size from the relatively small size grains to grains having dimensions such that the surface area of the cavity which supports current has less than crystallites of niobium per square inch as averaged over the entire current supportive surface of the cavity. Although heat treatment for two hours at 2000" C. is typically adequate to grow sufficiently large crystals, longer annealing times at higher temperature can yield up to a single crystal for constituting the entire surface area of the cavity.

In step (c), the cavity resonator structure having relatively large grains is then electropolished and tuned to the desired operating frequency by removing portions of the cavity structure or by deforming the walls of the cavity.

In step (d), the electropolished and tuned cavity structure is then assembled to produce a completed accelerator section or reference cavity.

In an alternative method of fabricating ultra high Q superconductive niobium cavities, the cavities are machined from a solid ingot 31 of niobium material having relatively large grains as indicated in FIG. 5. The large grained ingot 31 is obtained by slowly cooling an electron beam melted ingot. Such large grain material typically includes grains having axial lengths of 6 inches to 12 inches and 1 centimeter to 3 centimeters in characteristic transverse dimension. Such large grain material is commercially available from WahChang Corporation of Albany, Oregon. When the cavity resonators are machined out of the solid ingot 31, the ingot is typically bored to an inside diameter roughly indicated by dotted line 32 of FIG. 5. Loading discs 17 may be separately machined from the same material or machined in place out of the solid ingot 31. When the relatively large grained material is utilized as a starting material, the annealing step (b) in the aforedescri'bed method may be omitted. However, improved performance may be obtained by annealing the cavity resonator structure to relieve stresses and also to further increase the size of the grains. The cavity structure is then electropolished to remove work damage and to produce a smooth surface, tuned and assembled in accordance with steps (c) and (d) of the method of FIG. 4. When fabricating relatively long accelerator sections 2, the accelerator section is preferably made up of a number of sections of full and half-size cavities 3 and 3' which are then preferably joined together as by diffusion joints, welds or the like positioned midway along the length of the half length section cavities 3' in the manner indicated by joint 34 of FIG. 2. It is particularly desirable to place the joints 34 in the half length cavities 3 because the microwave currents associated with the 1r/2 accelerating mode are at a minimum in the half length section such that any loss associated wiih the joint 34 is minimized in use.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention can be made without departing from the scope thereof it is intended that all matter contained in the above description or shown in the accom- =panying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In an ultra high Q superconductive microwave circuit, means forming a cavity resonator structure made of niobium superconductive metal, the improvement wherein the superconductive niobium wall which defines the microwave current supportive surface of said cavity resonator structure is constituted by less than 20 crystallites of niobium per square inch as averaged over the entire current supportive surface of said cavity, whereby the radio frequency energy losses associated with grain boundaries in the surface of said cavity are minimized in use.

2. The apparatus of claim 1 wherein the ultra high Q superconductive circuit is a coupled cavity microwave linear particle accelerator section including, a plurality of superconductive cavities coupled together and arranged for interaction with a beam of charged particles passable through said coupled cavities to accelerate charged particles of the beam to nearly the velocity of light, and wherein a plurality of said coupled cavities have their microwave current supporting wall surfaces constituted of niobium having less than 20 crystallites of niobium per square inch as averaged over the entire current supportive surface of said cavities.

3. In the method for making an ultra high Q superconductive cavity resonator structure the steps of, forming the radio frequency current conductive wall surfaces of the cavity of niobium material, and heat treating the current supporting walls of said cavity resonator to anneal the niobium material and to produce growth of the niobium crystallites constituting current conducting walls of the cavity to a sufiicient extent such that the crystallites in the current supporting surface of the cavity are reduced in number to an average of less than 20 per square inch as averaged over the entire current supportive surface of the cavity resonator.

References Cited UNITED STATES PATENTS 2,899,598 8/1959 Ginzton 3l55.42 X 2,993,143 7/1961 Kelliher et al. 3155.42 X

H. K. SAALBACH, Primary Examiner S. CHATMON, IR., Assistant Examiner U.S. Cl. X.R. 

